Book Image

Learn LLVM 12

By : Kai Nacke
Book Image

Learn LLVM 12

By: Kai Nacke

Overview of this book

LLVM was built to bridge the gap between compiler textbooks and actual compiler development. It provides a modular codebase and advanced tools which help developers to build compilers easily. This book provides a practical introduction to LLVM, gradually helping you navigate through complex scenarios with ease when it comes to building and working with compilers. You’ll start by configuring, building, and installing LLVM libraries, tools, and external projects. Next, the book will introduce you to LLVM design and how it works in practice during each LLVM compiler stage: frontend, optimizer, and backend. Using a subset of a real programming language as an example, you will then learn how to develop a frontend and generate LLVM IR, hand it over to the optimization pipeline, and generate machine code from it. Later chapters will show you how to extend LLVM with a new pass and how instruction selection in LLVM works. You’ll also focus on Just-in-Time compilation issues and the current state of JIT-compilation support that LLVM provides, before finally going on to understand how to develop a new backend for LLVM. By the end of this LLVM book, you will have gained real-world experience in working with the LLVM compiler development framework with the help of hands-on examples and source code snippets.
Table of Contents (17 chapters)
Section 1 – The Basics of Compiler Construction with LLVM
Section 2 – From Source to Machine Code Generation
Section 3 –Taking LLVM to the Next Level

How instruction selection works

The task of an LLVM backend is to create machine instructions from the LLVM IR. This process is called instruction selection or lowering. Motivated by the idea to automate this task as much as possible, the LLVM developers invented the TableGen language to capture all the details of a target description. We first look at this language before diving into the instruction selection algorithms.

Specifying the target description in the TableGen language

A machine instruction has a lot of properties: a mnemonic used by the assembler and disassembler, a bit pattern to represent the instruction in memory, input and output operands, and so on. The LLVM developers decided to capture all this information in a single place, the target description. A new language, the TableGen language, was invented for this purpose. The idea was to use a code generator to create various source fragments from the target description, which could then be used in different tools...